Switch power sources are currently in the trend of evolving towards miniaturization at a high frequency, a high power density, a high efficiency and a low cost. Since semiconductor devices in traditional switch power sources function as hard switches, the sources suffer from a great loss, fail to improve their own efficiencies and thus become less and less competitive for the market due to their bulkiness. In view of limitations by the development in the industry of semiconductor devices, the cost, etc., a soft switch circuit topology has been an option for the majority of switch power source manufacturers to improve product competitiveness. There are numerous researches and patents on soft switch circuits, and one of them is a Auxiliary Resonant Commutated Pole (ARCP) soft switch circuit in the form of “inductor connected with switch in series”, which has won the popular favor of those skilled in the art of switch power sources due to its simply hardware circuit, easiness to control and satisfactory effect. Chinese Utility Model Patent ZL 200620131113.6, for example, discloses an ARCP soft switch circuit, which is an improvement of such a soft switch circuit.
FIG. 1 illustrates a schematic diagram of an operation principle of the soft switch circuit in the form of “inductor connected with switch in series” as follows:
Positive and negative direct current input voltage source±½Ud and primary power switching transistors SW1 and SW2 constitute a primary power half bridge inverter circuit, so that a high frequency pulse voltage of ±½Ud is generated at the point B by closing and opening SW1 and SW2 constantly, and a desired power frequency output voltage Uo is generated across a filter capacitor C3 and a primary power filter current I1 is generated across a filter inductor L1 by a primary power filter circuit. Here, the primary power switching devices SW1 and SW2 belonging to traditional hard switch circuits, which suffer from a great loss.
In order to decrease the loss of the primary power switching devices SW1 and SW2, two unidirectional auxiliary switching devices SW3 and SW4 and a resonant inductor L2 are added in the ARCP soft switch circuit, and the primary power switching devices SW1 and SW2 respectively are arranged in parallel across resonant capacitors C1 and C2 with large capacitances relative to parasitic capacitances of SW1 and SW2. A resonant current I2 is generated on the resonant inductor L2 in the same direction as the primary power filter current I1 by controlling the unidirectional auxiliary switching devices SW3 and SW4 to be closed and opened, and closing of the primary power switching devices SW1 and SW2 at a zero voltage is achieved by the resonance of the resonant inductor L2 and the resonant capacitor C3. Also the parallel arrangement of the resonant capacitors C1 and C2 with much larger capacitances than parasitic capacitances of the primary power switching devices SW1 and SW2 across SW1 and SW2 respectively achieves opening of the primary power switching devices at a zero voltage. Thus, the ARCP soft switch circuit can achieve both closing and opening of the primary power switching devices SW1 and SW2 at a zero voltage to thereby significantly decrease the loss of the primary power switching devices. Regarding the additional unidirectional auxiliary switching devices SW3 and SW4, no the sudden changes of current will occur due to the presence of the resonant inductor L2 in series therewith to thereby achieve closing at a zero current, and opening of the auxiliary switching devices SW3 and SW4 at a zero current can be achieved by controlling the moments of SW3 and SW4 to be closed and opened reasonably and effectively, so that the additional unidirectional auxiliary switching devices SW3 and SW4 can operate in a status of being both closed and opened at a zero current with a very small switching loss. FIG. 2A and FIG. 2B illustrate schematic diagrams of switching logics in positive and negative halves of a cycle respectively of this ARCP soft switch circuit.
As can be apparent from the foregoing analysis, the ARCP soft switch circuit achieve both switching of the primary power switching devices SW1 and SW2 at a zero voltage with a reduced loss and switching of the unidirectional auxiliary switching devices SW3 and SW4 at a zero current with a very small switching loss that substantially can be negligible to thereby archive a significantly improved overall operation efficiency, a greatly decreased overall loss, a markedly reduced volume and hence an enhanced competitiveness of the whole machine production.
Although the ARCP soft switch circuit is rather satisfactory in terms of the reduced loss of the switching devices and the improved efficiency, this circuit suffers from a significant drawback, i.e., imbalance of power output from the positive and negative direct current input voltage sources and consequent imbalance of positive and negative direct current input voltages, which may result in a series of problems, for example:    1) An excessive voltage may cause a device to be inoperative or damaged;    2) An output voltage may be asymmetry in positive and negative halves of a cycle, so indexes, e.g., precision, distortion, etc., of the output voltage will not be satisfactory.    3) A load fails to function normally.
In summary, a resonant current introduced in the ARCP soft switch circuit between the midpoint N of the positive and negative direct current input voltage sources±½Ud and the output point B of the half bridge circuit may cause inconsistency of output power of the positive and negative direct current input voltage sources±½Ud to thereby result in an offset of their voltages and further a series of problems. This may be an inevitable and fatal drawback of the ARCP soft switch circuit topology and consequently limit the application scope and reliability thereof.
In order to address this problem, it is a common practice to add an external balance circuit to balance the input voltages. As illustrated in FIG. 3, a balance current Ib introduced between the balance circuit and the midpoint N of the positive and negative direct current input voltage sources can cancel off the problem by controlling them to be equal to the resonant current I2, Ib=I2, that is, the resonant current I2 and the balance current Ib are both equal in magnitude and identical in direction at any time. For (I+)+(I2)=(I−)+(Ib), (I+)=(I−) can be derived from Ib=I2, and this indicates consistency of output power of the positive and negative direct current input voltage sources, thereby addressing the imbalance problem of the positive and negative direct current input voltage sources.
Although the foregoing method in which an external balance circuit is added can address the imbalance problem of the positive and negative direct current input voltages, it can be apparent that the entire circuit may be complicated and suffer from a raised cost, an increased volume, a lowered efficiency and greatly lowered reliability due to the additional balance circuit. In other words, an additional balance circuit has to be provided for the use of the ARCP soft switch circuit, and this balance circuit will scale up along with increasing power of the primary power circuit. If output power of the primary power circuit is tens to hundreds of kilowatts, the disadvantages of the additional balance circuit, such as volume, cost, efficiency, reliability, etc. will become more prominent and may even cancel off the benefit from the use of the ARCP soft switch circuit.